To purify spent fuel pool water from nuclear power plants and recycle the purified water as cooling water for spent fuel rods, a demineralizer using a granular ion exchange resin is placed as a purification device for fuel pool water. This demineralizer is placed to inhibit corrosion of stored spent fuels and various materials and remove radioactive substances from pool water, thus maintaining long-term soundness, such as decreased radiation exposure of operators.
In the demineralizer, it is necessary to replace ion exchange resins having degraded performance by fresh resins. In this case, since a volume of spent ion exchange resins are generated as a radioactive waste, the replacement costs money for the new ion exchange resins as well as money for disposal of the radioactive waste and requires a place for the disposal. For these reasons, it has been desired to prolong the lives of ion exchange resins.
However, spent fuel pool water that is obtained from a nuclear power plant such as a pressurized-water reactor (PWR) contains pro-oxidants such as hydrogen peroxide which is generated by decomposition of the water subjected to radiation from fuel rods and hydroperoxyl radicals and hydroxyl radicals which are generated from hydrogen peroxide (hereinafter, these pro-oxidants are referred to as “pro-oxidants”) and boron which is derived from boric acid added for control of nuclear fission reaction of fuels. In general, spent fuel pool water contains hydrogen peroxide in the order of a few or several ppm and boron in a concentration of about 2000 to about 3000 ppm. Such spent fuel water is treated directly by ion exchange in a purification apparatus for fuel pool water. However, a demineralizer using a granular ion exchange resins cannot remove those pro-oxidants. Hence, the pro-oxidants remain in fuel pool water, waste storage bunker water, and condensate storage water that is recovered after purification of fuel pool water or waste storage bunker water and then stored. In addition, since the pro-oxidants have a very strong oxidizing action, they oxidize cation resins in ion exchange resins and elute polystyrene sulfonic acid (PSS). The eluted PSS is attached to anion exchange resins and decreases their reaction rate. Further, hydrogen peroxide oxidizes and degrades cation exchange resins and, in consequence, sulfate ions and the like are eluted from the cation exchange resins and increase the electric conductivity at an outlet of an ion exchange resins column. The strong oxidizing action of the pro-oxidants contributes to corrosion of steel materials such as pipes and tanks.
It is believed that the main cause of the degradation of ion exchange resins is oxidation of cation exchange resins that is caused by their contact with pro-oxidants contained in such water. To solve this problem, the following methods have been proposed: a method of alkaline decomposition of pro-oxidant by contacting water containing the pro-oxidant with anion exchange resins before contacting the water with cation exchange resins (Patent Document 1: Japanese Patent Publication No. 2000-002787), a method of removing pro-oxidant by contacting it with granular active carbon and a method of removing pro-oxidant by contacting it with ion exchange resins on which platinum group catalyst particles are doped (Patent Document 2: Japanese Patent Publication No. H10-111387), a method of removing pro-oxidant by passing water containing the pro-oxidant through a platinum catalyst coated membrane (Patent Document 3: Japanese Patent Publication No. 2003-156589), a method of removing pro-oxidants by contacting them with active carbon to adsorb them (Patent Document 4: Japanese Patent Publication No. 2008-232773), and a method of removing pro-oxidants by passing water containing the pro-oxidants through a manganese filter (Patent Document 5: Japanese Patent Application No. 2012-217133). However, these methods proposed so far relate to purification of water having a low pro-oxidant concentration of about 0.01 to about 0.001 mg/L, such as nuclear reactor cooling water or radioactive waste water, and there are no examples of application of those methods to purification of spent fuel pool water containing pro-oxidants in a high concentration of 1 mg/L or more as well as boric acid (for example, about 2000 to about 3000 mg/L).